Space Elevator

Regarding actual space elevators though, while they're not sci-fi to the extent of something like FTL travel - ie. they're technically not physically impossible - they're still pretty firmly in the realm of sci-fi. We don't have anything close to a cable that could sustain its own weight, let alone that of whatever is being elevated. Plus, how do you stabilize the cable and lifter in the atmosphere?

A space elevator on the moon is much more feasible: less gravity, slow rotation, no atmosphere, less dangerous debris. But it's also much less useful.

The slow rotation is a minus, it means you've got to string the tether up to L1 instead of "just" up to geo/luna-stationary orbit. A lunar space elevator needs to be at least 56000 km long, more than 20000 km longer than the one to earth.

> But it's also much less useful.

Yeah, especially because all the things that make lunar space elevators a little more attainable also make lunar mass drivers a lot more attainable. Why ride in an elevator for a week if you also can just be fired from a cannon?

If the power building the space elevator wants to bomb you, you're going to get bombed.

1) How do you attach the climber to the cable without affecting its structural integrity? By squeezing it really hard? A material that's optimized for longitudinal tension strength is probably not very tolerant of lateral compression.

2) How do you provide power to the climber? A regular electric cable can't support its own weight, so either you have to attach it to the climbing cable, or you have to make it from the same material.

3) Is it even worth it? The climber needs to cover a distance of ~36,000 km, so even at 200 km/h it takes 7.5 days from the bottom to geosynchronous orbit. How many climbers and what payload can the cable support at the same time? Refer to issue #1 regarding limits in speed and mass per climber.

The throughput in tonnes/day is absolutely abysmal in relation to the immense upfront infrastructure cost per elevator. Compare this to SpaceX's Starship, which is getting closer and closer to fully reusable 100 tonnes to orbit in minutes. Space elevators will stay science fiction forever, not because they're infeasible, but because they're useless.

This is especially true considering that you don't need something that barely holds - you need something that you know will hold up to many times more weight than it needs to, so that it can be safe: the potential energy such a thing would store would be enough to dig into hundreds of meters of rock all around the world, if it ever crashed. So, you have to ensure there is no realistic chance of it ever crashing. It also has to be highly non-fragile in other ways, so that a madman with a bomb or a freak collision with an airplane or a meteor (especially likely in the thin upper layers of the atmosphere) won't bring it all down.

This combination of properties may well be completely impossible to actually achieve in a material. Even if there is no obvious basic law of physics that it would break, that doesn't mean that it wouldn't break other, harder to touch, derived laws.

Snipping off just the first few kilometers is not catastrophically destructive yet, and cutting it down further up would require multistage rocket designs, sophisticated steering/targeting and potentially significant yield (you'd need to cut unobtainium, after all...). If you can build a space elevator, you can defend against those.

You better thoroughly inspect what cargo you put on the elevator itself, of course.

https://foundation.fandom.com/wiki/Bombing_of_the_Star_Bridg...

It’s about as devastating as you would expect.

A terrorist attack on a space elevator is a pivotal plot point in Blue Mars by Kim Stanley Robinson, which IMHO is a better work in basically every way than Asimov's magnum opus.

I think it's the first episode of season 2 or 3, not the first season. I remember someone else mentioning it, but I've only seen season 1 and don't recall that either.

Pure payload capsules with no passengers wouldn't need this.

The argument for space elevators is that there's a pretty strong limit on how much payload can be launched by rockets due to injection of water into the upper atmosphere. Starship could arguably reach this limit with plausible projected growth rates in traffic.

Rotating cables ("rotavators") on the moon seem much more practical than full space elevators.

https://en.wikipedia.org/wiki/Momentum_exchange_tether#Rotov...

But yes, a space elevator would be difficult to defend in World War III.

When the ICBMs go up, early warning radars notice them right away and you still have time to act. Leaders can make it to helicopters and basement bunkers, bomber squadrons can scramble, missile silos can already be empty when hit, road mobile ICBM launchers can still relocate.

But with a large enough number of MRBMs, your opponent might get ideas. They might start thinking about getting away with a decapitation strike.

The military space elevator is more like an ICBM in this case. There will be ample warning when somebody drops something from geostationary orbit (and also when somebody drops something from lower up).

Because that's all they can do

The elevators were developed for cheap space travel but unsurprisingly centralized the world's economic development around the owner countries. ie the other countries became increasingly reliant on them and the world segmented into (three) blocs. But the owner countries became increasingly protective / paranoid, leading to cold-war era developments where each of them secretly researched fancy space weapons and stockpiled more and more military assets around the elevators.

So some of the attacks were by poorer countries lashing out. Some attacks were to expose the military assets being hidden in the elevator (outlawed by intl treaty). Though most were probably just excuses to show things like giant robots vs death star.

You look to be right:

https://www.kimstanleyrobinson.info/content/space-elevator

And I'm not the only one to notice the cross-reference:

https://www.reddit.com/r/kimstanleyrobinson/comments/pv6zh9/...

Not to mention, securing the cargo would be an extremely difficult task in itself, especially when one of the main thinga you'd like to raise through the space elevators is rocket fuel.

and what, 12" wide? 24"? that's still very difficult to target

If you account for various inefficiencies like taking it slow in the lower atmosphere Ant whatnot, it still should be in the matter of hours. So totally feasible and even comfortable.

This means that half-way after 58 minutes, the climber is traveling at 0.3 * 9.81 m/s² * 60 * 58 ~= 10.2 km/s ~= 36,720 km/h (!!!) relative to the cable. A tiny imperfection or wobble is going to make the climber crash into the cable, destroying both.

A climber with a mass of 10 tonnes requires 10^4 kg * 1.3 * 9.81 m/s² ~= 127.5 kN of force to accelerate at 1.3 g. At the ~56 minute mark, the climber reaches a speed of ~9,888 m/s. This means it requires a power output of 127.5 kN * 9888 m/s = 1.26 GW (!!!) to achieve this acceleration, plus overhead for the power electronics and transmission. Even at a voltage of 1 kV, that's around 1,500,000 A (!!!) of current that you have to transmit and invert.

If you have a way to reliably transfer that amount of power without touching the cable which is moving at 10 km/s relative speed, or with touching but without immediately melting the cable or the collector, let me know :-)

> So totally feasible

lol no

Maybe the Isp could be increased by mixing in some helium, but helium is very expensive.

They’re not obviously wrong.

A lot of the cable is moving at escape and orbital velocities. Tensile strength is all that holds it together.

If, as the cable fails, you sever the parts above from below around escape velocity, you’ll significantly reduce the length of cable that will ever hit the surface.

On Earth.

Zylon or M5 [1] could build an elevator on Mars. Kevlar on the Moon.

To drive this home, it’s estimated we could build a lunar space elevator for less than what Bechtel fleeced NASA for a mobile SLS launcher [2][3].

[1] https://en.wikipedia.org/wiki/M5_fiber

[2] https://opsjournal.org/DocumentLibrary/Uploads/The_Lunar_Spa...

[3] https://oig.nasa.gov/wp-content/uploads/2024/02/IG-22-012.pd...

A maglev train is several centimeters from the rail; if someone made the carbon nanostructures (the only known material strong enough are atomically precise carbon nanotubes or graphene, but the entire length has to be atomically precise you can't splice together the shorter tubes we can build today) this badly wrong, the cable didn't survive construction.

> Even at a voltage of 1 kV, that's around 1,500,000 A

Why on earth would you do one kilovolt? We already have megavolt powerlines. That reduces the current needed to 1500 A. 1500 A on a powerline is… by necessity, standard for a power station.

We even already have superconductor cables and tapes that do 1500 A, they're a few square millimeters cross section.

> Above GEO, the centrifugal force is stronger than gravity, causing objects attached to the cable there to pull upward on it. [...] On the cable below geostationary orbit, downward gravity would be greater than the upward centrifugal force, so the apparent gravity would pull objects attached to the cable downward.

So, without defensive countermeasures, the Space Elevator would indeed whip around the Earth.

But honestly, if I were designing such a thing, it would have break points, and maybe even a whinch at the base, to pull the line in. I'd also build it over water, and not over a population centre.

But I'm only a software engineer– it's likely a lot more challenging than this.

No maglev train I ever heard of travels at 36,000 km/h. This is about two orders of magnitude faster.

> We already have megavolt powerlines.

That's transmission over long distances, but you need to handle and transform all that power in a relatively small enclosure. Have you seen the length of isolators on high-voltage powerlines? What do you think is going to happen to your circuit if you have an electrical potential difference of 1 MV over a few centimeters?

Yes, you can handle large voltages with the right power electronics, but you need the space to do so. For comparison, light rail typically uses around 1 kV, while mainline trains use something like 15 kV. But a train is also 10 to 100 times as heavy as the 10t climber in my calculation, so you need to multiply the power (and therefore the electric current) by 10 to 100 as well.

You think the problem is the speed itself, and not the fact that trains are close to sea level and at that speed would immediately explode from compressing the air in front of them so hard it can't get out of the way before superheating to plasma, i.e. what we see on rocket re-entry only much much worse because the air at the altitude of peak re-entry heating is 0.00004% the density at sea level?

> What do you think is going to happen to your circuit if you have an electrical potential difference of 1 MV over a few centimeters?

1) In space? Very little. Pylons that you see around the countryside aren't running in a vacuum, their isolators are irrelevant.

2) Why "a few centimetres"? You've pulled the 10 tons mass out of thin air, likewise that it's supposed to use "one kilovolt" potential differences, and now also that the electromagnets have to be "a few centimetres" in size? Were you taking that number from what I said about the gap between the train and the rails? Obviously you scale the size of your EM source to whatever works for your other constraints. And, for that matter, the peak velocity of the cargo container, peak acceleration, mass, dimensions, everything.

> For comparison, light rail typically uses around 1 kV, while mainline trains use something like 15 kV.

Hang on a minute. I was already wondering this on your previous comment, but now it matters: do you think the climber itself needs to internally route any of this power at all?

What you need for this is switches and coils on one side, a Halbach array on the other. Coils aren't that heavy, especially if they're superconducting. Halbach array on the cargo pod, all the rest on the tether.

Right now, the hardest part is — by a huge margin — making the tether. Like, "nobody could do it today for any money" hard. But if we could make the tether, then actually making things go up it is really not a big deal, it's of a complexity that overlaps with a science faire project.

(Also, I grew up with 25kV, but British train engineering is hardly worth taking inspiration from for other rail systems, let alone a space elevator).

Compensate the slight loss of ISP by using aerospiked rotating detonation engines...

Just because it's moving below circular orbital speed doesn't mean the periapsis is in the atmosphere.

And it's why we have been so worried about Russian nukes--they have used liquid fueled birds, they can't be held ready to launch. Such birds are pretty much only useful for a first strike as they won't be able to launch them once incoming missiles are detected unless they're being held at launch ready (and they can't do that for too long.)

Climbing the cable is a nightmare, especially as it gets thicker as you go up. Thus do not climb the cable! Rather, when the cable is built a whole bunch of anchors are built into it. You are not climbing the cable, you are climbing a track on the side of the cable. The cable's job is to support the track plus any load on it.

Breakdown voltage is pressure dependent, not a constant.

Your figure is for (eyeballing a graph) approximately 2e-2 torr and 150 torr, less between, rapidly increasing with harder vacuum. The extreme limit even in a perfect vacuum is ~1.32e18 volts per meter due to pair production.

For a sense of "perfect" vacuum: if I used Wolfram Alpha right just now, the mean free path of particles at the Kármán line is about 15 cm, becomes hundreds of meters at 200km.

Though this assumes a free floating measurement, the practical results from https://en.wikipedia.org/wiki/Wake_Shield_Facility would also matter here.

> And you're operating this in space where you have ionizing radiation. Free electrons with a big voltage differential?

Mm.

Possibly. But see previous about mean free paths, not much actual stuff up there. From an (admittedly quick) perusal of the literature, the particle density of the Van Allen belts is order-of 1e4-1e5 per cubic meter, so the entire mass of the structure is only order-of a kilogram: https://www.wolframalpha.com/input?i=%284%2F3%29π%282%5E3-1%...

If this is an important constraint, this would actually be a good use for a some-mega-amps current, regardless of voltage drop between supply and return paths due to load. Or, same effect, coil the wires. And they'd already necessarily be coiled to do anything useful: Use the current itself to magnetically shield everything from the Van Allen belts.

Superconductors would only need a few square centimetres cross section to carry mega-amps, given their critical current limit at liquid nitrogen temperatures can be kilo-amps per mm^2.

But once you're talking about a 36,000 km long superconducting wire with a mega-amp current, you could also do a whole bunch of other fun stuff; lying them in concentric circular rings in the Sahara would give you a very silly, but effective, magnetic catapult. (This will upset a lot of people, and likely a lot of animals, so don't do that on Earth).

From what I've seen nobody currently directly launches more than 4.9 tons direct to GEO (Vulcan Centaur VC4). Starship is supposed do 27 to GTO (not GEO) when finished, but it's not finished.

If a space elevator lasts long enough to amortise the construction costs (nobody knows, what with them not being buildable yet), they would represent an improvement on launch costs relative to current methods, even if you were limited to 10 tons at a time and each GEO being a 2 hour trip.

And I don't understand the connection to the Van Allen belts--I'm talking about sunlight knocking electrons off your conductors.

I didn't say you did with that parenthesis, that was to indicate I was being very approximate with the pressures that correspond to your stated breakdown voltage: https://www.accuglassproducts.com/air-dielectric-strength-vs...

> I went looking for the dielectric strength of vacuum and I found a chart with values for a bunch of different things including vacuum.

That's even more wrong than looking up the value of acceleration due to gravity and applying "9.8m/s/s" to the full length of a structure several times Earth's radius (which was also being done in these comments).

Think critically: when you're reducing pressure, at what point does it become "a vacuum"? Answer: there is no hard cut-off point.

(Extra fun: https://en.wikipedia.org/wiki/Paschen%27s_law)

> And I don't understand the connection to the Van Allen belts

You mentioned free electrons. The thing Van Allen belts are, is fast-moving charged particles captured by Earth's magnetic field.

> I'm talking about sunlight knocking electrons off your conductors.

Very easy to defend against photoelectric emission.

Just to re-iterate, if you're lifting something up with a magnetic field, it's non-contact. You can hide the conductors behind any thin non-magnetic barrier you want and it still works.

Say, Selenium, with a work function of 5.9 eV. Tiny percentage of the solar flux is above that.

Even just shading them from the sunlight would work. Like, a sun-shade held off to one side.

Also, you could just have the return line inside the tether: If the supply is on the outside, return on the inside, you can even use the structure of the tether itself as shielding — coaxial voltage differential, so the voltage difference between supply and return lines due to load creates negligible external electrical field.

Honestly, this feels like you've just decided it won't work and are deliberately choosing the worst possible design to fit that conclusion. Extra weird as "but we can't actually build carbon nanotubes longer than 55 cm yet" is a great deal more important than all the stuff I've listed that we can do.